Sheet feeding device and image forming apparatus

ABSTRACT

Provided is a sheet feeding device and an image forming apparatus capable of performing sheet feeding by electrostatic adsorption at a low noise with a simple configuration. An adsorbing member  400  is arranged to be rotatable in synchronization with rotation of one driving roller (rotary member)  401 , and while the driving roller  401  is performing one rotation, the adsorbing member  400  adsorbs and feeds a sheet from a standby position, and then the adsorbing member  400  returns to the standby position again.

TECHNICAL FIELD

The present invention relates to a sheet feeding device and an image forming apparatus, and more particularly, to a technique of feeding a sheet using electrostatic adsorption force.

BACKGROUND ART

An image forming apparatus such as a copying machine or a printer according to a related art includes a sheet feeding device that feeds a sheet, and as the sheet feeding device, there is a friction feed system in which a topmost sheet is separated and fed from a cassette on which a sheet bundle is loaded using frictional force of a rubber roller or the like. In the sheet feeding device of the friction feed system, the topmost sheet is fed by the rubber roller rotating while pressing the sheet bundle. Here, when a sheet is fed, multi-sheet feeding in which a plurality of sheets are conveyed by friction between sheets may occur. On the other hand, conveyance resistance works on the remaining sheets excluding the topmost sheet through a separating pad or a retard roller, and thus only the topmost sheet is fed to an image forming portion.

Meanwhile, in the sheet feeding device of the friction separation system, since the rubber roller feeds a sheet while applying great pressure to the sheet, noise generated by sliding friction between sheets or between the sheet and the rubber roller is problematic. In addition, when the multi-sheet feeding caused by the separating pad or the retard roller is prevented, sliding fricative between sheets is greatly generated. Further, since the separating pad or the retard roller serves as conveyance resistance of the topmost sheet even when the multi-sheet feeding does not occur, a sound is generated by stick slip between the separating pad or the retard roller and the sheet.

In this regard, as a technique of solving the problem, there is a sheet feeding device configured to separate and feed a sheet while adsorbing the sheet using electrostatic adsorption force, specifically, by an electric field formed on a belt surface (see Patent Literatures 1 and 2). In the sheet feeding device of the electrostatic adsorption separation system, since it is possible to convey the topmost sheet as if the topmost sheet is peeled off from the sheet bundle, it is possible to significantly reduce noise generated in a feeding portion.

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Patent Laid-Open No. 2011-168396 -   Patent Literature 2: Japanese Patent Laid-Open No. 5-139548

SUMMARY OF INVENTION Technical Problem

However, in the sheet feeding device of the related art that feeds the sheet using electrostatic adsorption force, in a configuration of Patent Literature 1, it is possible to apply sufficient electrostatic adsorption force to the sheet, but when the sheet is separated, since lifting and lowering are performed for each frame on which the adsorbing belt is carried, an operation sound occurs. A collision sound with the sheet occurs as well. Further, when the sheet is adsorbed, belt tension is reduced by reducing an inter-axial distance so that a sheet can be adsorbed with certainty even when a sheet curls, that is, so that followability to the sheet curl can be secured when the adsorbing belt adsorbs the sheet. However, when the sheet is adsorbed in a state in which belt tension is reduced, it is necessary to increase tension at the time of the separation operation, and when the tension is increased as described above, string vibration occurs in the belt, and a sudden sound is caused by the vibration.

In a configuration of Patent Literature 2, the adsorbing belt is used, but since the sheet separation operation is performed by causing the carrying roller to perform an eccentric motion instead of lifting and lowering the adsorbing belt for each frame, a machinery operation sound is reduced. However, when the adsorbing belt comes into contact with the sheet bundle with certainty, the roller collides with the sheet bundle through the adsorbing belt, and thus a collision sound still occurs. Further, when an attempt to prevent a collision between the roller and the sheet bundle is made, the belt is separated from the sheet bundle, sheet adsorption by the adsorbing belt becomes unstable, leading to a feeding failure.

In this regard, in light of the foregoing, it is an object of the present invention to provide a sheet feeding device and an image forming apparatus, which are capable of stably performing sheet feeding by electrostatic adsorption at a low noise with a simple configuration.

Solution to Problem

The present invention provides a sheet feeding device, which includes a loading unit that loads a sheet, a rotary member arranged above the loading unit, an adsorbing member that is arranged to be movable in synchronization with rotation of the rotary member and electrically adsorbs the sheet loaded on the loading unit, a driving unit that rotates the rotary member, and a control unit that controls the driving unit, wherein the control unit causes the sheet loaded on the loading unit to be adsorbed on the adsorbing member by increasing a downward looseness amount of the adsorbing member, and then feeds the sheet adsorbed on the adsorbing member while reducing the downward looseness amount of the adsorbing member, and the control unit performs one rotation of the rotary member to move the adsorbing member from a standby position, causes the sheet to be adsorbed and fed, and then moves the adsorbing member to the standby position.

Advantageous Effects of Invention

According to the present invention, it is possible to cause the rotary member to perform one rotation to move the adsorbing member from the standby position, cause the adsorbing member to adsorb and feed the sheet, and then cause the adsorbing member to move the standby position. Thus, it is possible to stably perform sheet feeding by electrostatic adsorption at a low noise with a simple configuration.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a schematic configuration of an image forming apparatus equipped with a sheet feeding device according to a first embodiment of the present invention.

FIG. 2 is a diagram for describing a configuration of the sheet feeding device.

FIG. 3 is a diagram for describing a detailed configuration of an adsorbing member of a sheet adsorption separation feeding portion installed in the sheet feeding device and a generation principle of adsorption force by which the adsorbing member adsorbs a sheet.

FIG. 4 is a diagram for describing a voltage supply method of supplying a voltage to an adsorbing member of a sheet adsorption separation feeding portion installed in the sheet feeding device.

FIG. 5 is control block diagram of the sheet feeding device.

FIG. 6 is a diagram for describing a sheet separation feeding operation of the sheet adsorption separation feeding portion.

FIG. 7 is a timing chart of a time of sheet separation feeding of the sheet adsorption separation feeding portion.

FIG. 8 is a diagram for describing a configuration of a sheet feeding device according to a second embodiment of the present invention.

FIG. 9 is a diagram for describing a configuration of a sheet feeding device according to a third embodiment of the present invention.

FIG. 10 is a diagram for describing a detailed configuration of an adsorbing member of a sheet adsorption separation feeding portion installed in the sheet feeding device and a generation principle of adsorption force by which the adsorbing member adsorbs a sheet.

FIG. 11 is a diagram for describing a sheet separation feeding operation of the sheet adsorption separation feeding portion installed in the sheet feeding device.

FIG. 12 is a timing chart of a time of sheet separation feeding of the sheet adsorption separation feeding portion.

FIG. 13 is a diagram for describing a configuration of a sheet feeding device according to a fourth embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present invention will be described in detail with reference to the appended drawings. FIG. 1 is a diagram illustrating a schematic configuration of an image forming apparatus equipped with a sheet feeding device according to a first embodiment of the present invention.

In FIG. 1, 100 indicates an image forming apparatus, and 100A indicates an image forming apparatus body (hereinafter, referred to as an “apparatus body”). An image reading portion 41 that includes an image sensor of irradiating an original placed on a platen glass serving as an original placing platen with light and converting reflected light into a digital signal and the like is arranged above the apparatus body 100A. An original from which an image is read is conveyed on the platen glass by an automatic original feeding device 41 a. An image forming portion 55, sheet feeding devices 51 and 52 of feeding a sheet S to the image forming portion 55, and a sheet reversing portion 59 of reversing the sheet S and conveying the reversed sheet S to the image forming portion 55 are arranged in the apparatus body 100A.

The image forming portion 55 includes an exposure unit 42 and four process cartridges 43 (43 y, 43 m, 43 c, and 43 k) for forming toner images of four colors, that is, yellow (Y), magenta (M), cyan (C), and black (Bk). The image forming portion 55 further includes an intermediate transfer unit 44, a secondary transfer portion 56, and a fixing portion 57 arranged above the process cartridge 43.

Here, the process cartridge 43 includes a photosensitive drum 21 (21 y, 21 m, 21 c, and 21 k), a charging roller 22 (22 y, 22 m, 22 c, and 22 k), and a developing roller 23 (23 y, 23 m, 23 c, and 23 k). The process cartridge 43 further includes a drum cleaning blade 24 (24 y, 24 m, 24 c, and 24 k).

The intermediate transfer unit 44 includes a belt driving roller 26, an intermediate transfer belt 25 stretching to an inner secondary transfer roller 56 a or the like, and primary transfer roller 27 (27 y, 27 m, 27 c, and 27 k) that abuts the intermediate transfer belt 25 at a position opposite to the photosensitive drum 21. As will be described later, as transfer bias of a positive polarity is applied to the intermediate transfer belt 25 through the primary transfer roller 27, toner images having a negative polarity on the photosensitive drum 21 are sequentially multi-transferred onto the intermediate transfer belt 25. As a result, a full color image is formed on the intermediate transfer belt 25.

The secondary transfer portion 56 is configured with the inner secondary transfer roller 56 a and an outer secondary transfer roller 56 b that comes into contact with the inner secondary transfer roller 56 a with the intermediate transfer belt 25 interposed therebetween. Further, as will be described later, as secondary transfer bias of a positive polarity is applied to the outer secondary transfer roller 56 b, the full color image formed on the intermediate transfer belt 25 is transferred onto the sheet S.

The fixing portion 57 includes a fixing roller 57 a and a fixing backup roller 57 b. The sheet S is nipped and conveyed between the fixing roller 57 a and the fixing backup roller 57 b, and thus the toner image on the sheet S is pressed and heated, and then fixed onto the sheet S. The sheet feeding devices 51 and 52 include cassettes 51 a and 52 a, respectively, serving as a storage unit (loading unit) that stores the sheet S and sheet adsorption separation feeding portions 51 b and 52 b, respectively, having a function of feeding the sheets S one by one while adsorbing the sheet S stored in the cassettes 51 a and 52 a by static electricity.

In FIG. 1, 103 indicates a pre-secondary transfer conveyance path in which the sheet S fed from the cassettes 51 a and 52 a is conveyed to the secondary transfer portion 56, and 104 indicates a pre-fixing conveyance path in which the sheet S conveyed to the secondary transfer portion 56 is conveyed from the secondary transfer portion 56 to the fixing portion 57. 105 indicates a post-fixing conveyance path in which the sheet S conveyed to the fixing portion 57 is conveyed from a fixing portion 57 to a switching member 61, and 106 indicates a discharge path in which the sheet S conveyed to the switching member 61 is conveyed from the switching member 61 to a discharge portion 58. 107 is a re-conveyance path in which the sheet S reversed by the sheet reversing portion 59 is conveyed to the image forming portion 55 again in order to form an image on a reverse side of the sheet S having an image formed on one surface thereof by the image forming portion 55.

Next, an image forming operation of the image forming apparatus 100 having the above configuration will be described. When the image forming operation starts, the exposure unit 42 first irradiates the surface of the photosensitive drum 21 with laser beams based on image information provided from a personal computer (not illustrated) or the like. At this time, the surface of the photosensitive drum 21 is uniformly charged to a predetermined polarity and potential by the charging roller 22, and when the laser beams are irradiated, charges of a portion irradiated with the laser beams are attenuated, and thus an electrostatic latent image is formed on the surface of the photosensitive drum.

Thereafter, the electrostatic latent image is developed by yellow (Y), magenta (M), cyan (C), and black (Bk) toners supplied from the developing roller 23, and thus the electrostatic latent image is visualized as toner images. Then, the toner images of the respective colors are sequentially transferred onto the intermediate transfer belt 25 by primary transfer bias applied to the primary transfer roller 27, and thus a full color toner image is formed on the intermediate transfer belt 25.

On the other hand, in parallel with the toner image forming operation, in the sheet feeding devices 51 and 52, only one piece of sheet S is separated and fed from the cassettes 51 a and 52 a through the sheet adsorption separation feeding portions 51 b and 52 b. Thereafter, the sheet S is detected by sheet leading end detecting sensors 51 c and 52 c and reaches a pair of drawing rollers 51 d and 51 e. Further, the sheet S nipped between the pair of drawing rollers 51 d and 51 e is fed to the conveyance path 103 and abuts a pair of registration rollers 62 a and 62 b that is stopped, so that a position of the leading end thereof is adjusted.

Then, in the secondary transfer portion 56, the pair of registration rollers 62 a and 62 b are driven at a timing at which the full color toner image on the intermediate transfer belt matches the position of the sheet S. As a result, the sheet S is conveyed to the secondary transfer portion 56, and in the secondary transfer portion 56, the full color toner image is collectively transferred onto the sheet S through secondary transfer bias applied to the outer secondary transfer roller 56 b.

The sheet S onto which the full color toner image has been transferred is conveyed to the fixing portion 57 and receives heat and pressure in the fixing portion 57, and the toners of the respective colors undergo melting and color mixture and are fixed as a full color image to the sheet S. Thereafter, the sheet S to which the image has been fixed is discharged through the discharge portion 58 installed in the downstream of the fixing portion 57. Further, when an image is formed on both sides of the sheet, the conveyance direction of the sheet S is reversed by the sheet reversing portion 59, so that the sheet S is conveyed to the image forming portion 55 again.

Next, a configuration of the sheet feeding device 51 according to the present embodiment will be described with reference to FIG. 2. As described above, the sheet feeding device 51 includes the cassette 51 a and the sheet adsorption separation feeding portion 51 b that feeds the sheets S one by one while electrically adsorbing the sheet S stored in the cassette 51 a by static electricity. The sheet feeding device 51 further includes a lifting and lowering unit 301 that is installed to be lifted and lowered in the cassette 51 a and lifts and lowers a sheet supporting plate 301 a in which the sheets S are loaded and the sheet leading end detecting sensor 51 c that detects the passage of the sheet S fed by the sheet adsorption separation feeding portion 51 b.

The sheet adsorption separation feeding portion 51 b includes an adsorbing member 400, a driving roller 401, a driving unit 203, and a power source unit 205. The adsorbing member 400 has flexibility and has an endless shape larger than an outer circumference length of the driving roller 401, and the adsorbing member 400 has a cross sectional shape in which substantially a circular shape is maintained by its own elastic force. The driving roller 401 is rotatably shaft-supported by a shaft support member (not illustrated) whose arrangement position is fixed, and arranged with a predetermined gap Lr from the top surface of the topmost sheet Sa loaded on the cassette 51 a. Further, driving force from the driving unit 203 is transmitted to the driving roller 401 through the driving transmission unit. The sheet adsorption separation feeding portion 52 b arranged in the sheet feeding device 52 has the same configuration as the sheet adsorption separation feeding portion 51 b of the sheet feeding device 51, and thus a description thereof is omitted.

Here, in the present embodiment, a gap G is formed between the adsorbing member 400 and the driving roller 401, and a part of the adsorbing member 400 is fixed to a fixing portion 402 of the driving roller 401. In other words, the adsorbing member 400 is supported by the driving roller 401 in a loose state. The fixing of the adsorbing member 400 to the driving roller 401 may be performed using an adhesive or the like or may be performed by holding the adsorbing member 400 by a fixing member (not illustrated) and the driving roller 401. Further, when the adsorbing member 400 is fixed using the fixing member, the fixing member protrudes from the surface of the adsorbing member 400, but in this case, by fixing the adsorbing member 400 in a portion that does not come into contact with the sheet by the fixing member, the negative effects of the sheet feeding operation are prevented.

The lifting and lowering unit 301 includes a lifter 301 b that is installed to be rotatable down the sheet supporting plate 301 a, and changes the position of the sheet supporting plate 301 a and the position of a topmost sheet Sa loaded on the sheet supporting plate 301 a according to a rotation angle of the lifter 301 b. The sheet leading end detecting sensor 51 c is arranged in the sheet conveyance path between the sheet adsorption separation feeding portion 51 b and the pair of drawing rollers 51 d and 51 e. Success or failure of sheet feeding is detected by detecting whether or not the sheet leading end detecting sensor 51 c detects the sheet S at a predetermined timing. In the present embodiment, the sheet leading end detecting sensor 51 c is a non-contact reflective photo sensor, and detects the presence or absence of a detection target by irradiating the detection target with spotlight and measuring reflected light quantity thereof.

In FIG. 2, 302 indicates a plane of paper height detecting unit that detects the top surface position of the sheet S loaded on the sheet supporting plate 301 a. The plane of paper height detecting unit 302 is arranged above the sheet supporting plate 301 a and configured with a sensor flag 302 a and a photo sensor 302 b. The sensor flag 302 a is rotatably supported on a support portion (not illustrated), and one end of the sensor flag 302 a is arranged at a position at which it can come into contact with the top surface of the topmost sheet Sa, and the other end of the sensor flag 302 a is arranged at a position at which it can light-shield the photo sensor 302 b.

Here, when the top surface of the topmost sheet Sa is positioned at a predetermined height, the sensor flag 302 a rotates, and the photo sensor 302 b is light-shielded. A controller 70 of FIG. 5 which will be described later detects the position of the top surface of the topmost sheet Sa by detecting the light-shielding state of the photo sensor 302 b. The controller 70 controls an operation of the lifting and lowering unit 301 such that the top surface of the topmost sheet Sa is consistently detected by the plane of paper height detecting unit 302, and maintains the position of the sheet supporting plate 301 a to be a position at which the height of the top surface of the topmost sheet Sa is almost constant. As a result, a gap Lr between the driving roller 401 and the top surface of the topmost sheet Sa is maintained to be almost constant.

Here, in the present embodiment, when the adsorbing member 400 adsorbs and conveys the sheet, the adsorbing member 400 adsorbs the sheet by static electricity so that the sheets do not undergo sliding friction, and then the adsorbing member 400 is pulled upward while elastically deforming the adsorbing member 400. As the adsorbing member 400 is pulled upward while elastically deforming the adsorbing member 400 as described above, the sheet is separated from another sheet.

In this regard, in the present embodiment, the length of the adsorbing member 400 is decided so that a sheet contact area Mn illustrated in (a) of FIG. 6 (which will be described later) in which sheet adsorption force necessary for the adsorption separation is obtained is secured.

Further, the power source unit 205 includes a positive voltage supply unit 205 a serving as a first power source, a negative voltage supply unit 205 b serving as a second power source, and switches 205 c and 205 d that control voltage supply. The voltages generated by the power source unit 205 are supplied to the adsorbing member 400 through the driving roller 401, and the applied voltages generate the electrostatic adsorption force of attracting the sheet S in the adsorbing member 400.

Next, a detailed configuration of the adsorbing member 400 of the sheet adsorption separation conveying portion 51 b and a generation principle of the adsorption force by which the adsorbing member 400 adsorbs the sheet S will be described with reference to FIG. 3. FIG. 3 is a schematic diagram illustrating a configuration of the adsorbing member 400. (a) of FIG. 3 illustrates the surface of the adsorbing member 400, (b) of FIG. 3 illustrates a cross section of a power supply portion of the adsorbing member 400, and (c) of FIG. 3 illustrates a conceptual cross-sectional diagram of the electrostatic adsorption force working between the adsorbing member 400 and the sheet S.

The adsorbing member 400 includes a base layer 400 c, a positive electrode 400 a, and a negative electrode 400 b, and the positive electrode 400 a of the comb teeth shape and the negative electrode 400 b of the comb teeth shape are alternately arranged inside the base layer 400 c. In the present embodiment, the base layer 400 c is of polyimide serving as a dielectric having volume resistance of 108 Ωcm or more and has a thickness of about 100 μl. The positive electrode 400 a and the negative electrode 400 b are conductors having volume resistance of 106 Ωcm or less and made of copper having a thickness of about 10 μm.

Exposure regions 400 d and 400 e in which the positive electrode 400 a and the negative electrode 400 b serving as the other electrode are exposed are form on the back surface of the adsorbing member 400. A voltage from the positive voltage supply unit 205 a and a voltage from the negative voltage supply unit 205 b are supplied to the exposure region 400 d of the positive electrode 400 a and the exposure region 400 e of the negative electrode 400 b through the driving roller 201, respectively. As a result, in the present embodiment, the positive electrode 400 a and the negative electrode 400 b become the positive and negative voltages of about 1 kV, respectively.

An unequal electric field is formed near the surface of the adsorbing member 400 by the positive electrode 400 a and the negative electrode 400 b to which the voltage is applied. When the adsorbing member 400 approaches the sheet S, dielectric polarization occurs on the surface layer of the sheet S serving as a dielectric, and the electrostatic adsorption force is generated between the adsorbing member 400 and the sheet S due to Maxwell's stress.

The driving roller 401 includes an insulating shaft 401 c and conducting portions 401 a and 401 b attached to both end portions of the insulating shaft 401 c as illustrated in FIG. 4. The insulating shaft 401 c may be formed using resin having an insulation property or may be formed by performing insulation coating on a surface of a shaft made of metal. The conducting portions 401 a and 401 b may be formed by fixing a cylindrical shaped metallic cover to the insulating shaft 401 c or may be formed by patterning a conductive material around the insulating shaft 401 c. The conducting portions 401 a and 401 b come into contact with flat springs 406 a and 406 b, and the positive and negative voltages from the positive voltage supply unit 205 a and the negative voltage supply unit 205 b are supplied to the flat springs 406 a and 406 b, respectively.

In FIG. 4, 400 a indicates the positive electrode of the adsorbing member 400, 400 b indicates the negative electrode of the adsorbing member 400, 400 d indicates the exposure region in which the positive electrode 400 a is exposed, and 400 e indicates the exposure region in which the negative electrode 400 b is exposed. The adsorbing member 400 is fixed to the driving roller 401 so that the exposure regions 400 d and 400 e come into contact with the conducting portions 401 a and 401 b. Through this configuration, the positive voltage generated by the positive voltage supply unit 205 a is supplied to the positive electrode 400 a through the flat spring 406 a and the conducting portion 401 a, and the negative voltage generated by the negative voltage supply unit 205 b is supplied to the negative electrode 400 b through the flat spring 406 b and the conducting portion 401 b.

FIG. 5 is a control block diagram of the sheet feeding device 51 according to the present embodiment, and in FIG. 5, 70 indicates a controller. In addition to the sheet leading end detecting sensor 51 c, the plane of paper height detecting unit 302, and the like, the driving unit 203, the positive voltage supply unit 205 a, and the negative voltage supply unit 205 b, and the like are connected to the controller 70.

Next, the sheet feeding operation performed by the sheet adsorption separation feeding portion 51 b according to the present embodiment will be described with reference to FIG. 6. (a) of FIG. 6 illustrates an initial operation in which the adsorbing member 400 is arranged at the initial position of the feeding operation. At the time of this operation, for example, the controller 70 illustrated in FIG. 5 stops the driving roller 401 so that the adsorbing member 400 is separated from the sheet S by a predetermined gap Lr. In the present embodiment, the driving unit 203 is a step motor having an encoder, and a position at which a sensor (not illustrated) detects a home position of the encoder is the initial position of the feeding operation of the adsorbing member 400.

(b) of FIG. 6 illustrates an approach operation, and at the time of this operation, the controller 70 rotates the driving roller 401 in an arrow F direction. Here, in the present embodiment, a center Cb of the adsorbing member 400 is eccentric from the rotation center Cr of the driving roller 401. For this reason, when the controller 70 rotates the driving roller 401 in the arrow F direction, the adsorbing member 400 approaches the top surface of the topmost sheet Sa.

(c) of FIG. 6 illustrates a contact area increase operation, and at the time of this operation, the controller 70 increases a contact area Mc between the adsorbing member 400 and the topmost sheet Sa by rotating the driving roller 401 in the arrow F direction, similarly to the approach operation. In the present embodiment, since the driving unit 203 is the step motor, it is possible to determine the position serving as the contact area Mn by managing a rotation angle from the initial operation using the number of steps of the step motor. (d) of FIG. 6 illustrates an adsorption operation. At the time of this operation, after increasing the contact area up to Mn by the contact area increase operation, the controller 70 starts to apply the voltages from the positive voltage supply unit 205 a and the negative voltage supply unit 205 b to the adsorbing member 400, and thus the electrostatic adsorption force works between the adsorbing member 400 and the sheet S.

(e) of FIG. 6 illustrates a conveyance operation, and at the time of this operation, after the adsorption operation, the controller 70 continuously rotates the driving roller 401 in the arrow F direction. As a result, the leading end portion of the topmost sheet Sa adsorbed by the adsorbing member 400 is lifted in an arrow D direction and separated from a lower sheet Sb. In the present embodiment, a conveyance guide 303 is installed in the downstream of the driving roller 401 in the sheet feeding direction. The conveyance guide 303 guides the leading end portion of the topmost sheet Sa adsorbed on the adsorbing member 400 which is separated from the adsorbing member 400 due to the stiffness of the sheet Sa and conveys the topmost sheet Sa up to the pair of drawing rollers 51 d and 51 e.

(f) of FIG. 6 illustrates a separation operation, and at the time of this operation, the controller 70 stops the voltage supply to the adsorbing member 400 after the topmost sheet Sa being conveyed is nipped between the pair of drawing rollers 51 d and 51 e. As a result, the sheet Sa adsorbed and conveyed on the adsorbing member 400 is conveyed by the pair of drawing rollers 51 d and 51 e nipping the sheet Sa. Thereafter, when the driving roller 401 further rotates, the adsorbing member 400 is separated from the sheet Sa being conveyed and returns to the initial state illustrated in (a) of FIG. 6 in which it is on standby for conveyance of the next sheet Sb. Further, when the sheet Sa has not been detected by the sheet leading end detecting sensor 51 c, it is determined that there is a mistake in the feeding operation of the sheet S, the feeding operation is resumed starting from the approach operation. One sheet is fed from a plurality of sheets S loaded on the cassette 51 a through the above six processes. Further, it is possible to continuously feed the sheets S one by one by repeatedly performing the six processes.

FIG. 7 is a timing chart of the initial operation, the approach operation, the contact area increase operation, the adsorption operation, the conveyance operation, and the separation operation illustrated in FIG. 6. Here, a conveyance velocity u of the driving roller 401, a supply voltage vp from the positive voltage supply unit 205 a, a supply voltage vn from the negative voltage supply unit 205 b, and a detection pulse ps of the sheet leading end detecting sensor 51 c are illustrated. In FIG. 7, a zone from a time T0 to a time T1 indicated by P1 is an initial operation zone, the conveyance velocity u is set to 0, and the supply voltage vp and the supply voltage vn are set to 0. A zone from the time T1 to a time T2 indicated by P2 is an approach operation zone, and the conveyance velocity u is set to U.

A zone from the time T2 to a time T3 indicated by P3 is a contact area increase operation zone, and subsequently from the time T1, the conveyance velocity u is set to a velocity U. The time T3 is a point in time at which the adsorption operation is performed, and at this time, the supply voltage vp is set to +V, and the supply voltage vn is set to −V. A zone from the time T3 to a time T5 indicated by P4 is a conveyance operation zone, and continuously, the conveyance velocity u is set to U, the supply voltage vp is set to +V, and the supply voltage vn is set to −V. At a time T4 directly after the time T3, the detection pulse ps is output. Further, the controller 70 determines whether or not the feeding is retried according to whether or not the time T4 falls within a predetermined value range.

A zone from the time T5 to a time T6 indicated by P5 is a separation operation zone. The conveyance velocity u is set to U, but at the time T5, the supply voltage vp and the supply voltage vn are set to 0 again, and at the time T6, the conveyance velocity u is set to 0. A zone from the time T6 to a time T8 indicated by P6 is the initial operation zone, and preparation for feeding of the next sheet S is performed. In the initial operation zone P6, the sheet Sa adsorbed and conveyed on the adsorbing member 400 until now is conveyed by the pair of drawing rollers 51 d and 51 e arranged in the downstream. For this reason, the feeding of the next sheet can start after the time T7 at which the output of the detection pulse ps is stopped. Thereafter, the above operation is repeated, and thus continuous sheet feeding is performed.

As described above, in the present embodiment, the adsorbing member 400 is arranged to be rotatable in synchronization with the rotation of one driving roller (rotary member) 401. Further, the loaded sheet is adsorbed on the adsorbing member 400 by increasing the downward looseness amount of the adsorbing member 400, and then the sheet adsorbed on the adsorbing member 400 is fed while reducing the downward looseness amount of the adsorbing member 400. At this time, while the driving roller 401 perform one rotation, after the adsorbing member 400 adsorbs and feeds the sheet from the standby position, the adsorbing member 400 returns to the standby position again. As a result, it is possible to control the position of the adsorbing member 400 easily and accurately. Further, the adsorbing member 400 is movable to the adsorption position at which the adsorbing member 400 comes into surface contact with the sheet and adsorbs the sheet, the separation position at which the adsorbed sheet is separated from the lower sheet while eliminating the bending, and the separation position at which the adsorbed sheet is separated. In addition, the adsorbing member 400 rotates to adsorb the sheet and hands the adsorbed sheet over to the pair of drawing rollers 51 d and 51 e, and thereafter, the adsorbing member 400 is stopped at a position (a standby position) away from the sheet. Thus, it is possible to separate and feed the sheet without moving the frame carrying the adsorbing member 400, the driving unit, the roller, and the like. As a result, it is possible to stably performing sheet feeding by the electrostatic adsorption at a low noise with a simple configuration.

Further, in the present embodiment, the sheet S is separated and fed through the above operation processes, but the present invention is not limited to this example. For example, in the initial operation of the present embodiment, the driving roller 401 is stopped, but the driving roller may continuously rotate at a constant velocity from the separation operation according to a timing of the approach operation. Further, in the separation operation of the present embodiment, the voltage supply to the adsorbing member 400 is stopped, and the adsorbing member 400 is separated from the sheet S, but the voltage may be continuously supplied, and the sheet S may be conveyed by the adsorbing member 400.

In addition, in the present embodiment, the electrostatic adsorption force is generated between the adsorbing member 400 and the sheet S through the above-described configuration, but the present embodiment is not limited to this example. For example, the positive electrode 400 a and the negative electrode 400 b may not have the comb teeth shape and may have a shape of a uniform electrode in which the electric field can be formed between the electrodes 400 a and 400 b and the sheet S to dielectric-polarize the sheet S.

Next, a second embodiment of the present invention will be described. FIG. 8 is a diagram for describing a configuration of a sheet feeding device according to the present embodiment. In FIG. 8, the same reference numerals as those in FIG. 2 denote the same or corresponding parts.

In FIG. 8, 407 indicates a cylindrical sponge member serving as an elastic member installed between the driving roller 401 and the adsorbing member 400, and the sponge member 407 is eccentrically fixed to the outer circumference of the driving roller 401. The endless adsorbing member 400 is fixed to the outer circumference of the sponge member 407.

As the adsorbing member 400 is fixed to the outer circumference of the sponge member 407 as in the present embodiment, it is possible to fix the adsorbing member 400 easily with certainty and reduce the cost. Further, it is possible to prevent the adsorbing member 400 from being deformed to the driving roller side (the rotary member side). In the present embodiment, the endless adsorbing member 400 is fixed to the outer circumference of the sponge member 407, but the present embodiment is not limited to this example. For example, an open-ended adsorbing member may be fixed to a part of the sponge member, or an electrode having the same function as the adsorbing member may be arranged on the surface of the sponge member.

Next, a third embodiment of the present invention will be described. FIG. 9 is a diagram for describing a configuration of a sheet feeding device according to the present embodiment. In FIG. 9, the same reference numerals as those in FIG. 2 denote the same or corresponding parts.

In FIG. 9, 450 indicates an adsorbing member that includes no electrode thereinside and is made of resin. 251 c is a charging roller serving as a voltage applying member that is arranged above the adsorbing member 450 and presses the adsorbing member 450 downward. The charging roller 251 c is rotatably shaft-supported by a shaft support member (not illustrated) whose arrangement position is fixed and drivenly rotates with the movement of the adsorbing member 450. The AC power source 252 is connected to the charging roller 251 c, and charges are applied to the surface of the adsorbing member 450 through contact charging by the charging roller 251 c, and the electrostatic adsorption force of attracting the sheet S is generated by the applied charges.

Next, a detailed configuration of the adsorbing member 450 and a generation principle of the adsorption force by which the adsorbing member 450 adsorbs the sheet S will be described with reference to FIG. 10. (a) of FIG. 10 is a perspective view of the adsorbing member 450, and (b) of FIG. 10 illustrates a cross section of the adsorbing member 450.

The adsorbing member 450 is a member having a single layer structure made of resin and serves as a dielectric having volume resistance of 108 Ωcm or more. Further, an alternating voltage is applied from the charging roller 251 c pressed on the surface of the adsorbing member 450. As a result, a region charged to a positive polarity and a region charged to a negative polarity are formed on the surface of the adsorbing member 450 in a stripe form at intervals corresponding to the frequency of the AC power source 252 and the surface moving velocity of the adsorbing member 450 as illustrated in (a) of FIG. 10. An unequal electric field is formed near the surface of the adsorbing member 450 by the positive and negative charged regions alternately formed in the stripe form. Further, when the adsorbing member 450 in which the unequal electric field is formed as described above approaches the sheet S, dielectric polarization occurs on the surface layer of the sheet serving as a dielectric, and the electrostatic adsorption force occurs between the adsorbing member 450 and the sheet S by Maxwell's stress.

Next, a sheet separation feeding operation of the sheet adsorption separation feeding portion 51 b according to the present embodiment will be described. FIG. 11 is a schematic diagram chronologically illustrating an operation of feeding the sheet S through the sheet adsorption separation feeding portion 51 b.

(a) of FIG. 11 illustrates the initial operation of arranging the adsorbing member 450 at the initial feed operation position. At the time of this operation, the controller 70 illustrated in FIG. 5 stops the driving roller 401 so that the adsorbing member 450 is positioned at the initial position in which the adsorbing member 450 is separated from the charging roller 251 c by a predetermined gap Lc, and the adsorbing member 450 is separated from the sheet S by a predetermined gap Lb.

(b) of FIG. 11 illustrates the charging operation of causing the adsorbing member 450 to come into contact with the charging roller 251 c and charging the surface of the adsorbing member 450 by the alternating voltage from the AC power source 252, and at the time of this operation, the controller 70 rotates the driving roller 401 in the arrow F direction. Here, since the center Cb of the adsorbing member 450 is eccentric from the rotation center Cr of the driving roller 401, when the driving roller 401 is rotated from the initial state, the adsorbing member 450 starts to come into contact with the charging roller 251 c. Further, as the alternating voltage is applied while rotating the driving roller 401, a charged region Ch in which a positive polarity and a negative polarity are alternately charged is formed on the adsorbing member 450.

(c) of FIG. 11 illustrates the approach operation of causing the charged region Ch formed on the surface of the adsorbing member 450 to approach the top surface of the topmost sheet Sa, and at the time of this operation, the controller 70 continuously rotates the driving roller 401 in the arrow F direction. When the driving roller 401 is rotated as described above, since the center Cb of the adsorbing member 450 is eccentric from the rotation center Cr of the driving roller 401, the surface of the adsorbing member 450 approaches the top surface of the topmost sheet Sa.

(d) of FIG. 11 illustrates the contact area increase operation of causing surface of the adsorbing member 450 to come into surface contact with the topmost sheet Sa by performing the approach operation continuously and thus increasing the contact area Mc. At the time of this operation, the controller 70 rotates the driving roller 401 in the arrow F direction, similarly to the approach operation. As a result, the charged region Ch of the adsorbing member 450 comes into contact with the topmost sheet Sa, and the topmost sheet Sa is adsorbed on the adsorbing member 450. When the driving roller 401 further rotates, the contact area Mc between the charged region Ch of the adsorbing member 450 and the topmost sheet Sa increases.

(e) of FIG. 11 illustrates the conveyance operation of conveying the sheet Sa adsorbed on the charged region Ch of the adsorbing member 450 up to the pair of drawing rollers 51 d and 51 e while separating the sheet Sa from the loaded next sheet Sb when the contact area Mn is reached by the contact area increase operation. The contact area Mn indicates an area in which conveyance force of the adsorbing member 450 becomes conveyance force that overcomes conveyance resistance working on the topmost sheet Sa when the topmost sheet Sa adsorbed on the adsorbing member 450 is conveyed by the adsorbing member 450. Then, when the contact area reaches Mn as described above, the topmost sheet Sa starts to be conveyed toward the pair of drawing rollers 51 d and 51 e.

(f) of FIG. 11 illustrates the separation operation of separating the topmost sheet Sa being conveyed from the adsorbing member 450 after the topmost sheet Sa is nipped between the pair of drawing rollers 51 d and 51 e. When the sheet Sa is nipped between the pair of drawing rollers 51 d and 51 e, the charged region Ch of the adsorbing member 450 is away from the topmost sheet Sa, and thus the adsorption force of the adsorbing member 450 does not work. Then, when the adsorption force of the adsorbing member 450 does not work, the topmost sheet Sa being conveyed is conveyed by the pair of drawing rollers 51 d and 51 e between which the topmost sheet Sa is nipped.

Thereafter, when the driving roller 401 further rotates in the arrow F direction continuously, the adsorbing member 450 moves to the position at which the adsorbing member 450 is separated from the sheet Sa being conveyed and returns to the initial state in which it is on standby for conveyance of the next sheet Sb as illustrated in (a) of FIG. 11. Further, when the sheet Sa has not been detected by the sheet leading end detecting sensor 51 c, it is determined that there is a mistake in the feeding operation of the sheet S, and the feeding operation is resumed starting from the approach operation. One sheet is fed from a plurality of sheets S loaded on the cassette 51 a through the above six processes. Further, it is possible to continuously feed the sheets S one by one by repeatedly performing the six processes.

FIG. 12 is a timing chart of the initial operation, the charging operation, the approach operation, the contact area increase operation, the conveyance operation, and the separation operation illustrated in FIG. 11. Here, FIG. 12 is a diagram chronologically illustrating the conveyance velocity u of the driving roller 401, the supply voltage v from the AC power source 252, and the detection pulse ps of the sheet leading end detecting sensor 51 c. In FIG. 12, a zone from a time T0 to a time T1 indicated by P1 is the initial operation zone, and at this time, the conveyance velocity u is set to 0, and the supply voltage v is set to 0. A zone from the time T1 to a time T2 indicated by P2 is the charging operation zone, the conveyance velocity u is set to U, the supply voltage is set to an alternating voltage of ±V.

A zone from the time T2 to a time T3 indicated by P3 is the approach operation zone, and subsequently from the time T1, the conveyance velocity u is set to the velocity U, and at the time T2, the supply voltage v is set to 0 again. A zone from the time T3 to a time T4 indicated by P4 is the contact area increase operation zone, and the conveyance velocity u is continuously set to the velocity U. A zone from the time T4 to a time T6 indicated by P5 is the conveyance operation zone, and the conveyance velocity u is continuously set to the velocity U. The detection pulse ps is output at the time T5 directly after the time T4. The controller 70 determines whether or not the feeding is retried according to whether or not the time T5 falls within a predetermined value range.

A zone from the time T6 to a time T7 indicated by P6 is the separation operation zone. The conveyance velocity u is set to U, but at the time T7, the conveyance velocity u is set to 0. A zone from the time T7 to a time T9 indicated by P7 is the initial operation zone, and preparation for feeding of the next sheet S is performed. In the initial operation zone P7, the sheet Sa is conveyed by the pair of drawing rollers 51 d and 51 e. For this reason, the feeding of the next sheet can start after the time T8 at which the output of the detection pulse ps is stopped. Thereafter, the above operation is repeated, and thus continuous sheet feeding is performed.

As described above, in the present embodiment, it is possible to obtain the sheet adsorption force by charging the surface layer of the adsorbing member from the outside through the charging roller 251 c. As a result, since it is possible to charge the adsorbing member 450 without arranging the electrode inside the adsorbing member, it is possible to simplify the configuration of the adsorbing member 450 and reduce the cost.

Further, in the present embodiment, the sheet S is separated and fed by performing the feeding operation including the six processes, but the present invention is not limited to this example. A DC current may be connected to the charging roller 251 c to form a charged region in which an entire surface has a homopolarity without forming the positive and negative charged regions alternately on the adsorbing member 450. In this case, the electrostatic adsorption force per unit area is reduced, but the electrostatic adsorption force can be generated more conveniently.

Next, a fourth embodiment of the present invention will be described. FIG. 13 is a diagram for describing the sheet feeding device according to the present embodiment. In FIG. 13, the same reference numerals as those in FIG. 2 denote the same or corresponding parts.

In FIG. 13, 270 indicates an adsorbing member, and 271 indicates a driving roller. The driving roller 271 is rotatably shaft-supported by a shaft support member (not illustrated) whose arrangement position is fixed and arranged to be apart from the top surface of the topmost sheet Sa loaded on the cassette 51 a by a predetermined gap Lr. Further, driving force from the driving unit 203 is transmitted to the driving roller 271 through a driving transmission unit.

Further, a fixing member 272 for fixing the adsorbing member 270 is fixed to the driving roller 271. In the present embodiment, the adsorbing member 270 is an open-ended member having flexibility larger than the length of the circumferential surface of the driving roller 271, and both ends of the adsorbing member 270 are fixed to the fixing member 272 so that a substantially circular arc shape larger than a semicircular arc is drawn by its own elastic force.

As the adsorbing member 270 has an open-ended shape as in the present embodiment, it is possible to obtain the same effects as in the first embodiment, and it is possible to reduce the cost by reducing the length of the adsorbing member 270.

In the embodiment described so far, the sheet S is adsorbed on the adsorbing member by the electrostatic adsorption force, but the present invention is not limited to this example. For example, a fine fiber structure of a submicron order may be formed on the adsorbing member, and the sheet S may adsorbed by intermolecular attractive force working between the sheet S and the fine fiber structure.

REFERENCE SIGNS LIST

-   51, 52 Sheet feeding device -   51 a, 52 a Cassette -   51 b, 52 b Sheet adsorption separation feeding portion -   51 c Sheet leading end detecting sensor -   51 d, 51 e Pair of drawing rollers -   55 Image forming portion -   70 Controller -   100 Image forming apparatus -   100A Image forming apparatus body -   201 Driving roller -   203 Driving unit -   205 Power source unit -   205 a Positive voltage supply unit -   205 b Negative voltage supply unit -   252 AC power source -   270 Adsorbing member -   271 Driving roller -   272 Fixing member -   400 Adsorbing member -   400 a Positive electrode -   400 b Negative electrode -   401 Driving roller -   401 a, 401 b Conducting portion -   402 Fixing portion -   407 Sponge member -   450 Adsorbing member -   G Gap -   Mn Sheet contact area -   S Sheet -   Sa Topmost sheet 

1. A sheet feeding device, comprising: a loading unit that loads a sheet; a rotary member arranged above the loading unit; an adsorbing member that is arranged to be movable in synchronization with rotation of the rotary member and electrically adsorbs the sheet loaded on the loading unit; a driving unit that rotates the rotary member; and a control unit that controls the driving unit, wherein the control unit causes the sheet loaded on the loading unit to be adsorbed on the adsorbing member by increasing a downward looseness amount of the adsorbing member, and then feeds the sheet adsorbed on the adsorbing member while reducing the downward looseness amount of the adsorbing member, and the control unit performs one rotation of the rotary member to move the adsorbing member from a standby position, causes the sheet to be adsorbed and fed, and then moves the adsorbing member to the standby position.
 2. The sheet feeding device according to claim 1, wherein the adsorbing member is supported by the rotary member in a loose state.
 3. The sheet feeding device according to claim 1, wherein the adsorbing member is an endless member, and a part of the adsorbing member is fixed to the rotary member.
 4. The sheet feeding device according to claim 1, wherein the adsorbing member is supported by the rotary member through an elastic member attached to a circumferential surface of the rotary member.
 5. The sheet feeding device according to claim 1, wherein the adsorbing member is an open-ended member, fixed to a fixing member whose both ends are attached to the rotary member, and supported by the rotary member while forming a circular arc shape.
 6. The sheet feeding device according to claim 1, further comprising, a power source for applying a voltage to the adsorbing member and providing adsorption force of adsorbing the sheet by static electricity.
 7. The sheet feeding device according to claim 6, wherein two electrodes are arranged in the adsorbing member, and the power source includes a first power source that applies a positive voltage to one of the two electrodes and a second power source that applies a negative voltage to the other of the two electrodes.
 8. The sheet feeding device according to claim 6, further comprising, a voltage applying member that is arranged between the adsorbing member and the power source, abuts the adsorbing member before the adsorbing member comes into contact with the sheet, and applies a voltage from the power source to the adsorbing member.
 9. The sheet feeding device according to claim 8, wherein the power source is an alternating current (AC) power source.
 10. The sheet feeding device according to claim 1, wherein a magnitude of the adsorption force by the static electricity by which the adsorbing member eliminates looseness is set to a magnitude by which the sheet is separated from the adsorbing member due to stiffness of the sheet.
 11. The sheet feeding device according to claim 1, wherein the adsorbing member has flexibility and is arranged to be movable to a standby position at which the adsorbing member is away from the sheet loaded on the loading unit, an adsorption position at which the sheet loaded on the loading unit is adsorbed, a separation position at which the adsorbed sheet is moved upward and separated from a lower sheet, and a separation position at which the adsorbed sheet is separated from the adsorbing member.
 12. An image forming apparatus, comprising: an image forming portion that forms an image on a sheet; a loading unit that loads a sheet; a rotary member arranged above the loading unit; an adsorbing member that is arranged to be movable in synchronization with rotation of the rotary member and electrically adsorbs the sheet loaded on the loading unit; a driving unit that rotates the rotary member; and a control unit that controls the driving unit, wherein the control unit causes the sheet loaded on the loading unit to be adsorbed on the adsorbing member by increasing a downward looseness amount of the adsorbing member, and then feeds the sheet adsorbed on the adsorbing member while reducing the downward looseness amount of the adsorbing member, and the control unit performs one rotation of the rotary member to move the adsorbing member from a standby position, causes the sheet to be adsorbed and fed, and then moves the adsorbing member to the standby position. 